1. Field of the Invention
The present invention relates to a method for separating a liquid component from a solution containing two or more liquid components through the use of a permeable membrane, and more particularly to an improved method of separating the component in vapor-to-vapor permeation environment.
2. Description of the Prior Art
As a method of separating a particular liquid component from a solution or mixture such as an organic liquid containing two or more liquid components there has been developed a pervaporation (permeation evaporation) method. The principle of the permeation method utilizes a separation vessel divided into an upper solution chamber and a lower vacuum chamber by means of a permeable membrane. By reducing a pressure in the vacuum chamber while keeping the solution introduced into the solution chamber in contact with the membrane, a particular component in the liquid solution will permeate and diffuse through the permeable membrane selectively in preference to the other components so that the component to be separated selectively permeates through the membrane and evaporate from the surface of the permeable membrane into the vacuum chamber. In this permeation method, however, because of that the liquid solution is kept in direct contact with the permeable membrane to effect the permeation in a liquid-to-vapor phase environment, there arises a serious problem that the permeable membrane which is generally made from a macromolecular material is likely to be swelled due to the direct contact with the solution. When the permeable membrane is swelled, its permeability will be considerably degraded to thereby lower the separation efficiency to an unacceptable level.
To overcome the above problem, the inventor has proposed in U.S. patent application Ser. No. 130,504 [U.S. Pat. No. 4,983,303] and Ser. No. 363,021 an evapomeation (evaporation permeation) method which is directed to effect the permeation in vapor-to-vapor phase environment. That is, as schematically shown in FIG. 1, the evapomeation method utilizes a like vessel 10 divided into a lower solution chamber 12 and an upper vacuum chamber 13 by a non-porous permeable membrane 14 and effects the permeation separation while keeping the solution S from which a particular component is to be separated out of contact with the permeable membrane 14. Thus, the vapors of the particular component will selectively permeate through the membrane 14 in a vapor phase into the vacuum chamber 13 when drawn by vacuum through the membrane 14. With this result, the evapomeation method can be free from the swelling which is seen in the conventional pervaporation method and therefore prevents a lowering in separation efficiency. Nevertheless, the evapomeation method may be found insufficient due to limited separation efficiency inherent to the permeable membrane employed. That is, since the separation relies upon diffusion permeation of the component into the membrane which occurs only at a relatively slow rate, it is not expected to separate the liquid components at a high rate sufficient for industrial application.